The invention relates to a rotor, which in operation is flown through by a fluid in a main flow direction, the rotor having a rotor blade arranged rotatable around a rotor axis and extending at least partially away from the rotor axis into the fluid, the rotor blade being split into at least two partial blades at a predetermined distance from the rotor axis, one partial blade being curved in a turning direction away from the rotor blade and the other partial blade being curved against the turning direction away from the rotor blade, the two partial blades being connected to form a loop.
From the prior art, rotors for the generation of thrust or propulsion or for the generation of an angular momentum are known and comprise in the first case propellers and marine propellers, blowers, fans, and ventilators etc., and, in the second case, flow-driven repellers, turbines and windmills. In the case of marine or aeronautic propellers, a rotor blade mounted on a hub is turning around an axis of revolution and generates a propulsion force due to its profile shape or due to its incidence while rotating around the axis of revolution. Averaged over one revolution, the propulsion force extends essentially parallel to the axis of revolution and propels the ship or the plane. In the case of helicopter rotors, a shifting of the rotor blades during the rotation around the axis of revolution may generate a propulsion force, which is inclined relative to the axis of revolution. In this context, the main flow direction is understood as the direction, under which the flow passes the rotor if the rotor is reduced to a plane in a far-field view.
The efficiency of rotors is lessened by flow losses in the form of vortex generation, swirl, andxe2x80x94if the rotor is operated in liquid mediaxe2x80x94by cavitation. Often, sound emission is a problem. The noise generated by marine or aeronautic propellers, by helicopter rotors, wind mills, various fans and blowers, for example in air conditioning systems, contributes significantly to today""s acoustic environmental load.
From the prior art, rotors of the general type are known that, in comparison to previously known rotors, lead to an improved efficiency and to a reduction of the sound emission or of the sound generation.
For example, a rotor is known from DE 42 26 637 A1 having a rotor blade, which is split into two partial blades. This rotor may reduce the vibrations during operation.
Another rotor is known from DE-PS 83050. This rotor leads to an increase in the reaction pressure.
In U.S. Pat. No. 1,418,991 a rotor is described having a rotor blade that is split into two partial blades at a distance from the axis of revolution, the partial blades extending in and against the direction of rotation of the rotor with respect to the rotor blade. The rotor of U.S. Pat. No. 1,418,991 may reduce the flow resistance.
Rotors of this general type are also known from U.S. Pat. No. 3,504,990 and from U.S. Pat. No. 4,445,817.
The rotor shown in U.S. Pat. No. 3,504,990 has support arms that do not affect the flow. At the ends of the support arms there is mounted an annular surface, which generates the propulsion of the rotor over its circumference or perimeter.
From U.S. Pat. No. 4,445,817, there is known a rotor having rotor blades that are made from a planar rotor stand extending perpendicular to the main flow direction and from a strip-shaped rotor blade. Each strip-shaped rotor blade is bent and fastened tot the rotor blade coming next in the turning direction. This rotor, too, generates propulsion exclusively at the outer perimeter of the rotor blade.
Further, in DE 197 52 369 a propulsive body is shown that has an end lying transverse to the flow direction and being split while forming a loop.
In EP 0 266 802 a curved-surface propeller is shown, of which the conveying blades form sections of an envelope of a cone. The curved surface and the conveying blades each surround holes through which the conveyed mass flows.
The disadvantages of the rotors known from the prior art is that the improvements they achieve in the efficiency and in the noise emission are no longer sufficient for today""s applications.
Therefore, it is the objective of the present invention to improve the initially mentioned rotors by simple design measures so that their efficiency is increased.
In particular, it is intended that, besides having an improved efficiency, the rotor generates less noise and is thus particularly beneficial to the environment.
This object is solved by a rotor of the initially mentioned type on one side in that a propulsion force or an angular momentum around the rotor axis is generated in operation by the rotor blade, and in that the rotor axis passes through the loop area enclosed by the loop.
On the other side this object is solved for a rotor of the initially mentioned type in that a propulsion force or an angular momentum around the rotor axis is generated in operation by the rotor blade, and in that the leading edge of the one, front, partial blade is situated in the main flow direction upstream of the leading edge of the other, rear, partial blade at least in an area close to the rotor blade.
This solution is simple and results in a significant noise reduction with the rotors.
By the connecting or combining the partial blades to a loop, the circulations changes continuously along the blade contour from one partial blade to the other. The circulation must be zero at one point along the blade contour, where the sign of the circulation between the two partial blades changes. Thus, the loop forms a means by which the circulation of the rotor over the whole perimeter of the loop and therefore in the trailing vortices generated in the wake is distributed uniformly. Thereby, a more even distribution of the circulation may be reached along the rotor blade and along the loop-forming partial blades than is known in the prior art. The vorticity is distributed spatially over the whole blade perimeter or circumference of the loop, which leads to decreased losses due to vortex generation and to a decreased generation of flow noise.
The loop-shaped ring closure of the partial blades also results in a high mechanical stability of the rotor. This may reduce the structural weight and may lead to an altogether more filigree design. Further, the depth or chord of the profile may be reduced at those parts of the loop formed by the partial blades, which parts only make a small contribution to the thrust. This may reduce the frictional resistance.
If, in the case of a single-bladed propeller, the axis of revolution passes through the plane of the loop, imbalances during the rotation of the propeller may be avoided by this simple design measure.
For simple applications, such as simple fans or ventilators or toy planes or toy wind wheels, the rotor blade and the partial blades may be formed for example as inclined flat planes; for technically more complex applications, such as airplane propellers or ship propellers, the rotor blade and the partial blades may be formed by wing-like profiles with a particular thickness distribution and camber. A profile that, depending on the local flow conditions, changes along the rotor blade and along the partial blades, leads to particularly advantageous flow characteristics and to an improved efficiency. In this context, the change of the blade depth and of the local angle of attacks may be adapted to the local flow conditions.
The generation of thrust or propulsion, or of an angular momentum by the rotor blades themselves is essential to the invention. By this principle, the inventive rotor differs from the rotor of U.S. Pat. No. 4,445,817. Although this rotor may form a loop, the rotor disclosed there does not have rotor blades that extend away from the axis of revolution into the flow and generate thrust. Rather, the loop-shaped ribbon is held by support arms, which must interact with or disturb the fluid flow as little as possible. The rotor of U.S. Pat. No. 4,445,817 generates the propulsion solely by the circumferential ribbon. In operation, the ribbon performs a movement similar to the wave motion of a fish tail or of a bird wing. This way of propelling has a low efficiency. The rotor according to the invention also differs by the above-mentioned principle from other known loop designs, such as the blade apparatus of U.S. Pat. No. 5,890,875. In this design, there are again provided no rotor blades, which leads to the generation of only small propulsion forces or angular momentums, respectively.
In the embodiment according to the invention, one partial blade may be in front of the other partial blade, both with respect to the turning direction and the main flow direction, in at least an area close to the rotor blade. In this case, either the leading edge of the one partial blade may be located upstream with respect to the main flow direction of the leading edge of the other partial blade, or the one partial blade may be located as a whole upstream of the other partial blade. In this arrangement of the partial blades, the flow from the pressure side of the front, upstream partial blade may be directed to the suction side of the rear, downstream rotor blade at least in the region close to the rotor blade. Thus, kinetic energy is added to the flow around the downstream partial blade by the faster flow from the low-pressure or suction side of the upstream partial blade. This leads to a more stable flow around the rear partial blade.
In a preferred embodiment of the rotor, the partial blades may be gradually bent away from the rotor blade in an essentially transition-free or jump-free manner. Such a continuous shape transition of the partial blades may avoid corner flows that may otherwise be present at kinks or folds during strongly varying operational conditions of the rotor, and may lead to losses.
In a further very advantageous embodiment, both partial blades may be integrally connected at their ends. Due to the mechanical connection of the partial blades, this design is particularly stable and capable of bearing loads, thus being able to reduce the vibrations generated by the rotor. Furthermore, the danger of injuries, which is caused by the tips of the partial blades, is minimized.
In a further advantageous embodiment of the invention, both partial blades may preferably smoothly transition or merge into each other. This means that both partial blades have, at their point of connection, essentially the same profile shapes and that their contours are connected in an essentially smooth manner with each other. This may for example be accomplished by an integral formation of the two connected partial blades.
The flow losses may be independently minimized in that in another advantageous embodiment, the trailing edge of the rotor blade continues as the trailing edge of the rear, downstream partial blade, preferably in a smooth and continuous manner. Likewise, the leading edge of the rotor blade may continue as the leading edge of the front, upstream rotor blade. By this, it is avoided that the flow is adversely affected by irregularities in the leading edge. Similarly, the distribution of the circulation may change considerably if only small irregularities are present at the trailing edge of the corresponding rotor blade or partial blade, respectively. In another embodiment, which is particularly advantageous in the case of a rotor according to the invention having only a single rotor blade, the axis of revolution may pass through the loop area.
In a further advantageous embodiment, the rotor may be provided with a plurality of rotor blades that are preferably equidistantly spaced in the turning direction and have the front, upstream partial blade of a rotor blade connected respectively with the rear, downstream partial blade of a rotor blade being located ahead in the circumferential direction. These embodiments make sure that there is low material consumption and a reversion of the circulation along the loop. The larger number of rotor blades makes possible an altogether larger propulsion at a smaller volume. The advantage of this embodiment lies in the combination of high propulsion or a high angular momentum with an advantageous uniform distribution of the circulation in the wake of the rotor. In spite of its high output or power, the rotor remains silent in this embodiment. Preferably, the partial rotors of rotor blades that are adjacent in the circumferential direction are connected to form a loop.
To obtain optimum flow conditions at the rotor blade and at the partial blades in various operational states, at least one of the rotor blades and the partial blades should be adaptable to a change in the local flow conditions, i.e., to flow conditions that are limited to the location of the respective rotor blade and/or partial blade. For this, at least one of the rotor blade and the partial blade may be at least sectionwise provided with an elastic outer skin. Using a proper tuning or adaptation of the materials and a corresponding pretensioning of the outer skin, local changes of the profile geometry of the rotor blade and/or the partial blade are possible either solely because of the fluid mechanical forces acting on the profile, i.e., passively, or by using a contour adaptation means, i.e., actively, without the occurrence of folds or kinks at the outer skin that may negatively influence the flow. Furthermore, an elastic outer skin with a properly selected elasticity may lead to a flow around the rotor blade and/or the partial blade having fewer losses. An elastic outer skin may react to local pressure disturbances by locally rather limited deformations and thus may absorb these local pressure disturbances, which will result in a more silent as well as less noise emitting flow around the rotor blade and/or the partial blade.
Further, the rotor may be equipped with a profile adjustment means in another advantageous embodiment. The profile adjustment means may act on the outer skin and may be shiftable at least sectionwise for a local or even global change of the profile geometry of the rotor blade and/or the partial blade. In this context, a local change is understood as a change in the profile geometry or in the contour of the rotor blade and/or the partial blade, respectively, that takes place only in a rather limited area of the rotor blade and/or the partial blade and leaves the flow essentially unaffected in other areas of the rotor blade and/or the partial blade. A global change of the profile geometry, in contrast, changes a large portion of the profile geometry of the rotor blade and/or the partial blade and leads to a substantial change of the characteristics of the flow around the rotor blade and/or the partial blade.
In a further advantageous embodiment, the rotor may be equipped with a hub, at which the rotor blade is supported. The rotor blade may be supported rotatable or pivotable at the hub by a means for changing the angle of attack. By changing the angle of attack, the propulsion generated by the rotor may be held constant over a wide range of rotational velocities of the rotor or may be adapted to the momentary operational conditions in a very simple manner. In an elastic design of the rotor, it is further possible to obtain a favorable torsion or wash-in of the blade structure along the loop lines. Likewise, a means for adjusting the angle of attack may be provided, by which the partial blade is rotatably supported at the rotor blade for changing the angle of attack. In fluid mechanics, the angle of attack in general stands for the inclination of the profile chord of the rotor blade and/or the partial blade relative to the local on flow of the rotor blade and/or partial blade. The profile chord connects the leading edge, i.e., the connecting line of the upstream stagnation points of the rotor blade and/or partial blade, with the trailing edge, i.e., the connection of the downstream stagnation points of the rotor blade and/or the partial blade.
In a further advantageous embodiment, a yaw or means for changing the sweep angle may be provided by which the rotor blade is held pivotable with respect to the hub in the direct of the propulsion. By such an actuating means, the sweep angle of the rotor blade, i.e., the angle of the leading edge relative to the main flow direction may be changed and the circulation generated by the rotor may be better distributed in the wake. A similar means may also be provided between the rotor blade and the partial blade to change the sweep of the partial blade. To take the swirling component of the on flow of the rotor blade and/or the partial blade into consideration when the sweep angle is being changed, the sweep angle changing means may pivot at least one of the rotor blade or the partial blade about a flow parallel to the axis of revolution.
In a further advantageous embodiment, a means for changing an opening angle may be provided between the rotor blade and the partial blade, with which at least one partial blade is connected in such a manner that an opening angle between two partial blades of a rotor blade may be adapted depending on the operational state of the rotor, said opening angle being directed essentially in the direction of rotation.
A further possibility for adaptively adjusting the rotor geometry, which improves the efficiency at a large number of operational situations, may be reached by an extension means, which is provided between the rotor blade and the partial blade and by which the partial blade is supported extendable with respect to the spanwise direction of the rotor blade. By extending the partial blades and/or the rotor blade, the surface area that generates the propulsion is enlarged so that more propulsion may be generated while keeping the circulation per unit surface area of the rotor blade and/or the partial blade constant.
Finally, two or more rotors according to the invention may be coupled in series. If the rotors rotate in opposite directions relative to each other, the corresponding vorticity strengths are superimposed in the wake of these two rotors and partially cancel each other. Using an appropriate adjustment, even a complete cancellation of the vorticity strength in the direction of the axis of the revolution, the swirl, of at least one of the rotors may be reached. By the elimination of the swirl, the losses may be minimized in the wake of the propellers connected in series. An optimum superposition of the vorticity strengths may be reached if the rotors have approximately the same diameter. The upstream rotor may also be configured as a stator, thereby reducing the design efforts.